Added regression test for bimolecular reaction syntax.

When parsing bimolecular reaction syntax '<->', MCell did not properly
initialize the flag indicating if the product was a complex or not for
the reverse reaction pathway. This led to reactions being incorrectly
identified as complex resulting in a crash. Thanks a lot to Sabina
Hrabetova for bringing this to our attention. This was fixed with
commit ID 0eca73ee751 in MCell's master branch.